The basic problem is neutral-ground, neutral-isolated ground and isolated ground-ground faults. In all circuits there can be neutral-ground faults, when isolated ground is present, there can also be neutral-isolated ground and isolated ground-ground faults. Hot-neutral and hot-ground faults are far more easily detected than neutral-ground, neutral-isolated ground, and isolated ground-ground faults, since current will usually trip circuit breakers or similar protective devices. Neutral and ground (and isolated ground, when present) should only be connected by the ground bond at the transformer or source panel in theory according to electrical codes. Ground conductors are not intended to carry load currents, while neutral conductors are. Once electric circuits are installed the only present effective method of testing for neutral-ground, neutral-isolated ground and isolated ground-ground faults is disconnecting part or all of a circuit and testing it bit by bit, first to detect neutral-ground, neutral-isolated ground, and isolated ground-ground faults and then to locate them. This is time consuming, so expensive, needing at least an apprentice electrician, and requires disconnecting initially all, then parts of an electrical system downstream of a transformer or other power source. Such a system can cover part of a floor, a floor, a building, a block, sometimes more. Such interruption is unacceptable to users of the system in any event, and impossible with computer systems, computer controlled systems and most medical equipment, however necessary checking the system for faults may in fact be.
Ground is always used to provide a low impedance path to the earthed point of the supply in case of a hot or neutral conductor fault. It may also be used to provide a zero-voltage low impedance ground reference or noise free background for sensitive instruments, such as computers and the like. When there is a neutral-ground fault, current flows in the ground conductor compromising the zero-voltage reference function. Two ground conductors are often present, the low impedance path to ground is provided by a safety ground conductor, while the zero-voltage reference is provided by an isolated ground conductor. Both ground conductors are joined to the neutral at the electrical source of the system, the transformer, and there connected to ground or preferably directly to earth.
Generally the isolated ground conductor is used as a quiet ground reference for equipment, insulated from the ordinary grounded parts of the circuit, and is not designed as a safety ground. If the isolated ground conductor is accidentally connected to ground at a receptacle, outlet or in equipment, the features of the isolated ground circuit are lost to the whole system. There are no overt indications of a ground-isolated ground fault or contact. Both ground and isolated ground (and indeed neutral) conductors are bonded together at the source of the electrical service and connected to earth at that point, it is difficult to detect an improper connection (or fault).
Isolated ground or neutral to ground faults typically occur in wiring boxes where the neutral or isolated ground conductor inadvertently makes a connection with another conductor. Most frequently, this fault can occur through slicing of conductor insulation, or local compression from a mounting screw or receptacle ground strap, or improper placing of the bare safety bonding conductor. Less frequently the fault may be caused by a nail or screw puncture of the conduit or cable, or a rough point in a conduit. The fault can occur over a period of time by cold flow through the insulation at the point under pressure, so that failure may occur in circuits after they have been put in service, as well as at the time of construction. Another cause is improper connections between equipments so the isolated ground of one equipment is connected to ground of another equipment.
As discussed under prior art, circuit testing is necessarily performed at installation. The method of the invention can be incorporated with standard circuit testing at installation. Before about 1970 (pre-electronic) electrical equipment was not sensitive to power fluctuations. Electronically controlled equipment (solid state, digital, computers, etc.) caused the use of electricity to expand greatly, leading to variability of power demand, modified electrical supply and so to power quality problems. Now the switching of electric appliances “on/off” affects power quality in that general circuitry. Modern circuit design accommodates power quality. Power quality testing is essential, at installation and later, in many modern circuits and is meaningless in the presence of ground-neutral, isolated ground-neutral and isolated ground-ground faults, thus testing for such faults is a pre-requisite for power quality testing, and currently no comprehensive method exists. Both circuit and power quality testing are required for hospitals and similar institutions.
For instance an intact ground will usually have a voltage of less than 20 millivolts, in the presence of a neutral-ground fault the voltage may be from 2 to 10 volts. If that ground is connected to an intracardiac lead the higher voltage can be fatal.
The method of the invention can be used to find broken neutrals and grounds, neutral breaks are known and can be and are often catastrophic, in multi-phase systems the full current flow between phase hot lines can fry electrical equipment.
Maintenance of power quality is vital to operation of computer systems. UPS (Uninterrupted Power Supplies) are integral to most power systems involving computers for this reason. UPS vary through small power boxes with battery/transformer/power supply charger inverter converting battery supply to 60 Hertz, for computers up to big systems for radio or telephone systems, with their own generators. UPS are 99.9% reliable. Besides normal power failures, ground breaks and neutral breaks are common enough to warrant precautions against such failures.
All these facts indicate that power circuits require live testing at installation for circuit integrity and power quality, as well as later live testing to find faults. At present there is no method to do this.